1. Field of the Invention
The present invention concerns a switching circuit for an electromagnetic source for the generation of acoustic waves of the type having a capacitor that is switched in parallel with at least one series circuit composed of another capacitor and a first diode.
2. Description of the Prior Art
A switching circuit for an electromagnetic pressure wave source of the above type is known from German OS 198 14 331. It has two LC oscillators connected in series. Of these, the first switching circuit has a first capacitor and, in parallel to this, a semiconductor power switch formed by a triggerable thyristor and a recovery diode switched antiparallel to the thyristor, as well as a subsequent inductance. Part of this first switching circuit, switched in series with the semiconductor power switch and the inductance, as well as parallel to the first capacitor, is a second capacitor that likewise belongs to the second switching circuit. Connected parallel to it is a saturable inductor and an electromagnetic pressure wave source fashioned as an inductive load. As soon as the thyristor of the semiconductor power switch has been triggered in the conductive state, the first capacitor charged with the capacitor charge device is connected to the second, initially uncharged capacitor, such that its charge passes into the second capacitor. The inductor and both capacitors are dimensioned such that the saturable inductor goes into saturation (and thus is of low inductance) only at the point in time when practically the same charge has been loaded from the first capacitor to the second capacitor. At this moment, due to the discharge voltage of the second capacitor with a time constant predetermined by the second switching circuit, a high discharge current flows through the inductive load of the electromagnetic pressure wave source, where an acoustic pulse is generated.
The switching circuit disclosed in Soviet Union 17 188 patent for the inductivity of an electrodynamic radiator has a common voltage source to which are connected a number of parallel branches with, respectively, one diode at the input, a storage capacitor connected to ground and an output-side commutator, i.e. switch. The diodes are thereby polarized such that the storage capacitors of the individual parallel branches always remain separated (i.e. unconnected) with regard to their charge voltages, such that transfer or transient effects of these charge voltages among one another are prevented. At the mutual discharging of storage caps, the commutators of all parallel branches are collectively, i.e. simultaneously, closed. During this discharging event, the storage capacitor of the respective branch is switched in parallel to its input-side diode.
A further switching circuit according to the prior art is shown in FIG. 1. The switching has a direct voltage source 1, a switch 2 that is normally executed as a discharger, a capacitor C as well as a coil L that is part of a sound generating unit of the electromagnetic source. In addition to the coil L, the acoustic wave generation unit of the electromagnetic source has a coil carrier (not shown) upon which the coil is arranged and an insulated membrane (likewise not shown) arranged on coil L. Upon the discharge of capacitor C via the coil L, a current i(t) flows through coil L, whereby an electromagnetic field is generated that interacts with the membrane. The membrane is thereby repelled in an acoustic propagation medium, whereby source pressure waves are emitted in the acoustic propagation medium as a carrier medium between the acoustic wave generation unit of the electromagnetic source and a subject to be acoustically irradiated. Shock waves can arise, for example, via non-linear effects in the carrier medium of the acoustic source pressure waves. The design of an electromagnetic source, especially of an electromagnetic shock wave source, is, for example, specified in European Application 0 133 665, corresponding to U.S. Pat. No. 4,674,505.
Shock waves are used, for example, for non-invasive destruction of calculi inside a patient, for instance for the destruction of a kidney stone. The shock waves directed at the kidney stone produce cracks in the kidney stone. The kidney stone finally breaks apart and can be excreted in a natural fashion.
If the switching circuit shown in FIG. 1 is operated for the generation of acoustic waves, during the discharge event of the capacitor C via the coil L (for which a short circuit is generated by means of the switch 2) the curves of the voltage u(t) (exemplarily plotted in FIG. 2) (curve 3) over the coil L and of the current i(t) (curve 4) result via the coil L. The decaying current i(t) flowing through the coil 4 is, as mentioned already, causes the generation of acoustic waves.
The acoustic waves generated by the electromagnetic shock wave source are proportional to the square of the current i(t) (curve 5 in FIG. 2). Subsequently originating from the discharge event of the capacitor C are a first acoustic source pressure wave from the first acoustic source pressure pulse (1st maximum) and further acoustic source pressure waves from the abating sequence of positive acoustic source pressure pulse. The first source pressure wave and the subsequent source pressure waves can, as mentioned already, form into shock waves with short, intensified positive portions and subsequently long, negative pressure troughs via non-linear effects in the carrier medium and a non-linear focusing which normally ensues with a known acoustic focusing lens.
Via the frequency of the current i(t) flowing through the coil L, characteristics of the shock wave (such as, for example, its focal radius) can be altered. With a variable current frequency, and thus a variable frequency of the shock wave, the size of the effective focus can, for example, be modified and adjusted to the subject to be treated dependent on the application. For instance, in a lithotripter the effective focus can be selected corresponding to the respective stone size, such that the acoustic energy is utilized better for the disintegration of the stone and the surrounding tissue is stressed less.
Due to the relatively high short circuit capacity up to the 100 MW range, a variable capacitance of the capacitor C and a variable inductance of coil L are costly. In order to vary the shock wave, in generally only the charge voltage of the capacitor C is therefore varied, whereby the maxima of the current i(t) changes via the coil L and the voltage u(t) to the coil L. However, the curve shapes of the current i(t) and the voltage u(t) remain essentially the same.